Two of the more popular office-type prior art sheet retainers used in binding a stack of punched-paper sheets together are the so-called ACCO prong and Duo-Tang tang binders shown in FIGS. 1A and 1B hereof. In the ACCO binder, FIG. 1A, a pair of prongs extending from a longitudinal base are inserted through spaced apertures from one side or top margin of a paper sheets stack. A keeper, sometimes called a compressor bar, with rectangular apertures is placed over the prongs from the opposite side of the stack so that the prongs pass through the apertures. The prongs are then bent 90.degree. toward each other to rest in a longitudinal groove in the keeper and a pair of locking loops slidable along the groove then are positioned over the bent prongs to temporarily lock the prongs on the keeper. The paper stack is thus held temporarily at a margin by being pressed between the prong base and the keeper. The fastener is normally made of metal, is relatively thin and can become easily twisted, has edges which may cut a hand or which may scratch a fine surface on which it is laid. Further it extends above the paper sheets stack, can become disconnected inadvertently during use and does not have a pleasing appearance. In addition, since often 3-hole punched paper is held in such a binder by the prongs passing through only the first and third holes (leaving the center hole unsecured), present ACCO binders tend to bow in the middle when the covers are opened, exposing the center hole, likewise presenting an unpleasing appearance. In the Duo-Tang binder, FIG. 1B, usually three separate or interconnected pairs of tangs are inserted through a paper stack normally with an associated cover. The tangs normally are constructed as a pair of legs integrally extending from a grommet pre-clamped on a first margin portion of a paper sheets cover having a second cover margin spaced therefrom containing a second open grommet. The paper stack is placed between the two margins with the tang(s) bent 90.degree. to the margin, passed through the paper sheets aperture(s) and the second grommet or aperture in the second separate or integral cover margin, and the tangs rebent 90.degree. away from each other to rest against the second margin. As in the FIG. 1A device, the tangs are usually metal, can scratch or cut, can be inadvertently rebent outwardly or twisted, bound sheets may likely be easily pulled out and the binding has an unpleasing exposed look.
Broadly, others including a co-inventor of this invention have proposed binding systems which at least in part overlie or encapsulate a head of a fixed post and socket-type of paper sheets connector and which is aesthetically pleasing. These prior inventions are exemplified by U.S. Pat. Nos. 4,743,048; 4,072,326; 4,139,216; 3,834,739; and 3,730,560. To the knowledge of the inventors herewith means such as described herein have not been suggested to improve a prong-type or tang-type fastener binding of the ACCO or Duo-Tang type. U.S. Pat. No. 4,548,426 shows a three-piece binding cover in which portions of one piece overlie common staples used to assemble the binding.